The present invention relates to a method and apparatus for preheating material with the hot gas being exhausted from a heater or kiln. In particular, the present invention relates to a preheating method and apparatus which more efficiently uses the energy of the hot gas to uniformly heat particulate material, even if the particulate material is not entirely uniform in itself.
Preheaters are commonly used for preheating many types of particulate material. One common use for preheaters is for preheating limestone particulate material. The limestone particulate material is generally preheated by inducing hot exhaust gases from a rotary calcining kiln through the limestone particulate material prior to placement of the limestone particulate into the calcining kiln. The gases heat the limestone particles prior to their introduction to the rotary kiln, and less heating is required in the rotary kiln to complete the calcining process. The preheater thus makes the entire calcining process more efficient and saves energy. Preheating apparatuses of this general type are known and described in prior art patents including U.S. Pat. Nos. 3,601,376; 3,832,128; 3,903,612; 4,337,031 and the prior art discussed and cited therein.
Several preheaters use a countercurrent heat exchange relationship, wherein the hot exhaust gas is directed opposite to the direction of flow of the particulate material. The countercurrent heat exchange relationship places the hottest exhaust gas against the warmest section of the particulate material, and vice versa, such that efficient heating occurs throughout the preheater.
In using a preheater, the limestone is typically supplied by conveyor to an overhead storage bin positioned above the preheater. The preheater may be located over a rotary kiln. In a preheating apparatus such as that disclosed in U.S. Pat. No. 4,337,031, an annular preheating passage extends between the overhead storage bin and a central discharge which is in communication with the rotary kiln. As the limestone is directed downwardly through the preheating passage, hot exhaust gases from the kiln move upward through the limestone particulate material.
While preheaters snake limestone calcining and other similar processes more efficient, advances in preheater design can be made to obtain further benefit, make the preheater more efficient, and save even more energy.